Long haul high capacity fiber optics communication systems require high performance light emitting sources capable of generating single mode, narrow spectral linewidth radiation in the 1.3-1.56 .mu.m wavelength range. Some of the existing semiconductor lasers, for example, InGaAsP DFB lasers can meet requirements for high power and proper wavelength, but a high dynamic single mode yield is difficult to achieve.
Conventional index coupled DFB lasers employing an index corrugation have an inherent problem in existence of two longitudinal modes across a laser stop band, a right Bragg mode and a left Bragg mode, having equal threshold gain. It inevitably results in poor single mode operation as shown, for example, in the article by H. Kogelnik and C. V. Shank "Coupled-mode theory of distributed feedback lasers", J. Appl. Phys., vol. 43, no. 5, pp. 2327-2335, 1972. One of the known approaches to the solution of this problem is asymmetric coating of laser facets, as shown, for example, in the publication by K. Utaka, S. Akiba, K. Sakai and Y. Matsushima "Effect of Mirror Facets on Lasing Characteristics of Distributed Feedback InGaAsP/InP Laser Diodes at 1.5 .mu.m Range". Asymmetric coating of facets of the laser results in asymmetric losses for two degenerate longitudinal modes, and consequently in different threshold gains for these modes. The final single mode yield in such lasers, however, is relatively low because of the random facet phases introduced by cleaving. Another approach to single mode generation is incorporation of a quarter-wave phase shift along a direction of propagation of one of the two degenerate longitudinal modes, as shown, for example, in the artfile by K. Utaka, S. Akiba, K Sakai, and Y. Matsushima ".lambda./4-Shifted InGaAsP/InP DFB Lasers". Although this type of DFB lasers with perfect anti-reflection (AR) coatings can theoretically provide up to 100% of a single mode yield, the yield of these lasers deteriorates rapidly with small changes in reflectivity of two modes. Usually, even a few percent difference in mode reflectivity can invalidate a single mode generation completely. Furthermore, the output power of these lasers is relatively low. Due to the laser symmetry, almost half of the power is emitted and wasted from one of the other laser facets. Moreover, a phenomenon known as a spatial hole-burning, the reduction of carrier density caused by high photon density in the center of a laser cavity, substantially deteriorates a side mode suppression ratio and the output power of such lasers at high current injection levels.
An alternative approach to breaking the mode degeneracy in DFB lasers is an introduction of gain coupling mechanism as predicted by H. Kogelnik and C. V. Shank (see the reference on page 1). In complex coupled (gain coupled or loss coupled) DFB lasers, a periodic optical gain or loss modulation in the presence of conventional index corrugation along the laser cavity effectively breaks the mode degeneracy between the two Bragg modes around the stop band of the DFB lasers, and thus avoids a serious and inherent problem for conventional index coupled DFB lasers, as predicted by E. Kapon, A. Hardy, and A Katzir in the publication "The Effect of Complex Coupling Coefficients on Distributed Feedback Lasers". These pure gain coupled and complex coupled DFB lasers have been demonstrated, for example, in publications by Y. Luo, Y. Nakano, K. Tada et al. "Purely gain-coupled distributed feedback semiconductor lasers", Appl. Phys. Lett., vol. 56, pp. 1620-1622, 1990, and G. P. Li, T. Makino, R. Moore et al. "Partly gain-coupled 1.55 .mu.m strained layer multi-quantum well DFB lasers", IEEE J. Quantum Electronics, vol. QE-29, pp. 1736-1742, 1993. With introduction of even a small amount of gain or loss coupling, the dynamic single mode yield of complex coupled DFB lasers increases drastically. It effectively provides lasing predominantly on a preferred and fixed Bragg mode among the two originally degenerate ones around a stop band, regardless of random distribution of unknown laser facet phases. In in-phase gain coupled DFB lasers, the higher index region within a grating period has a higher optical modal gain, resulting in lasing mainly of the right Bragg mode with the lasing wavelength longer than the Bragg wavelength. Both theory and experiments have confirmed this conclusion. For anti-phase loss coupled DFB lasers, the higher index region within a grating period experiences higher optical loss, resulting primarily in lasing of the left Bragg mode with the lasing wavelength shorter than the Bragg wavelength.
Complex coupled DFB lasers have been studied intensively by different scientific groups. It has been demonstrated they have many advantages over conventional index coupled lasers, such as high single mode sensitivity, less sensitivity to external reflections, high modulation bandwidth and reduced wavelength chirp. That is why it is considered that these lasers constitute a good candidate for practical applications in fiber optics communication systems. Some of the recent achievements are reported in the following publications: W. T. Wsang, F. S. Choa, M. C. Wu et al. "Semiconductor distributed feedback lasers with quantum well superlattice gratings for index or gain-coupled feedback", Appl. Phys. Lett., vol. 60, pp. 2580-2582, 1990 and J. Hong, K. Leong, T Makino et al. "Impact of random facet phase on modal properties of partly gain-coupled DFB lasers", J. Selected Topics on Quantum Electronics, vol. 3, no. 2, pp. 555-568, 1997.
Among other complex coupled DFB lasers, multiple quantum well (MQW) lasers with etched quantum wells appear to provide the highest performance operation up to date. Additionally, by utilizing compressively strained MQW structure to enhance the differential gain, the speed of the devices has been increased substantially. For example, the article by G. P. Li, T. Makino, R. Moore and N. Puetz "1.55 .mu.m index/gain coupled DFB lasers with strained layer multiquantum-well active grating", Electronics Letters, vol. 28, no. 18, pp. 1726-1727, 1992, describes a partly gain coupled (mixed index and gain coupled) DFB laser, having a MQW active grating. The grating is patterned by etching grooves through all MQW layers of the active region and regrowing InP buffer material in the etched grooves. The high corrugation region in the groove, having more quantum wells, provides a higher modal gain and real modal index than the low corrugation region in the groove, thus, providing gain and index coupling mechanisms. However, an external quantum efficiency of this type of laser is limited by high index coupling, losses in the groove regions where there is no gain medium, and leakage current from the bottom of the grooves to the n-side confinement region. Another MQW DFB laser with improved characteristics is described in the article by H. Lu, C. Blaauw, B. Benyon et al. "High-Power and High-Speed Performance of 1.3-mm Strained MQW Gain-Coupled DFB Lasers", IEEE J. of Selected Topics in Quantum Electronics, vol. 1, no. 2, pp. 375-381, 1995. The laser comprises a MQW grating structure similar to that described above with the only difference that the grating grooves are patterned through the several top quantum well layers only, and some of the bottom quantum well layers are not etched. This type of the DFB laser exhibits higher quantum efficiency due to existence of some gain medium in low corrugation regions in the grooves.
Nevertheless, an efficient control of coupling coefficients is still limited. Fabrication of these lasers is quite complicated, because it requires high precision controlled etching through the active region to avoid damage to the active layers. Additionally, it is difficult to control gaih and index coupling independently, because both ipdex and gain coupling are determined by the etching depth.
Therefore there still exists a continued demand for the development of alternative structures of the DFB semiconductor lasers, providing simplified fabrication process, while providing high essential parameters of the lasers.